This invention relates generally to polyphase power supplies and more particularly to apparatus for preventing damage to loads caused by incorrectly wiring such loads or polyphase electrical equipment coupled to such loads to the polyphase power supply.
Certain equipment, such as scroll compressors, are designed to rotate in a given direction and will be severely damaged if caused to rotate in the reverse direction. In the case of a scroll compressor even a single partial rotation in the reverse direction will permanently damage the scroll plates. It is typical to use three phase motors to drive such compressors and, if the motor phase sequence is miswired, reverse rotation will result. That is, with respect to a three phase motor having windings A, B and C, spaced 120.degree. apart, if these windings are coupled to phases A, C, B, respectively of a three phase power supply, the motor will run in a direction reversed to that intended.
It is therefore important that the wiring sequence be verified prior to energization of such a compressor system. A known method for detecting phase miswiring in a three phase system utilizes two neon indicator lamps and a capacitor. The resistance of the neon lamps and the reactance of the capacitor are chosen to be approximately the same and are connected in a Y configuration with the lamps coupled to what are believed to be phases A and C and the capacitor coupled to phase B. If in fact the phase sequence is as intended the second lamp coupled to phase C will glow due to the voltage drop of approximately 1.37.times.V.sub.Line. The voltage drop across the other, first, lamp will be approximately 0.37.times.V.sub.Line, below the level required for illumination. On the other hand, if in fact the phase sequence is miswired, i.e., A-C-B, the voltage drop across the first lamp will be approximately 1.37.times.V.sub.Line and it will glow and the second lamp will be approximately 0.37.times.V.sub.Line, below that voltage level required for illumination thereby indicating a miswired condition.
Although this method can be used effectively, it does not represent a satisfactory solution to the problem. The possibility exists of someone servicing the compressor, or associated equipment, requiring the disconnection and subsequent reconnection of the leads, either not having the neon lamps and capacitor or of making an error by misinterpreting the results. Even during initial set-up of the installation the compressor system is frequently out of the direct control of the manufacturer of the system and therefore the manufacturer is not in a position to ensure that tile wiring will be done correctly. Since the cost of the compressor can be up to thousands of dollars, it would be very desirable to obviate the possibility of damage to the system occasioned by miswiring of the phase sequence.